In certain oilfield applications, pump assemblies are used to pump a treatment fluid from the surface down a wellbore, often at extremely high pressures. Such applications include but are not limited to hydraulic fracturing, cementing, acidizing, and pumping through coiled tubing, among other applications. In the example of hydraulic fracturing operations, a multi-pump assembly is often employed to direct a fracturing fluid into the wellbore and to a selected region(s) of the wellbore. The configuration of the well bore can vary and is subject to the type of completion most effective for the particular situation. The fracturing fluid is pumped from the wellbore into the formation to create “fractures” connecting the native reservoir to the wellbore that intersects the reservoir or the fracture network. To create such fractures, the fracturing fluid is pumped at pressures ranging from 1,000 to 15,000 psi or more. The mass flow rate of the fracturing fluid will vary depending upon what is required for the wellbore conditions. In addition, the fracturing fluid may or may not contain a propping agent, hereinafter called “proppant”. The proppant is used to keep the fracture “propped” open after the creation of the fracture as well as aiding in other fracturing mechanisms. These fractures provide communication pathways to the reservoir and allow formation deposits to flow into the wellbore and to the surface of the well. These additional pathways serve therefore to enhance the production of the well.
A power pump is typically employed for conveying the fracturing fluid into the wellbore during fracturing operations. A power pump is a positive displacement pump consisting of one or more cylinders each containing a piston or plunger. Such pumps are sometimes also referred to as positive displacement pumps, intermittent duty pumps, triplex pumps, or quintuplex pumps. This style of pump translates rotating motion to a linear actuation by means of a crankshaft—slider mechanism. The plunger is moved in two directions along a single axis. This motion moves the plunger in and out of a chamber in a pressure housing (typically referred to as a fluid end) in order to change the fluid volume of the chamber. Fluid enters the chamber through a one way valve as the plunger is sliding out of the chamber and the chamber volume increases. As the plunger slides into the chamber and decreases chamber volume, the fluid is displaced out of the chamber through a one way valve. This pumping action occurs in each of the fluid chambers of the fluid end and the summation of the individual compartments combines for a total output flow from the fluid end.
Multiple pumps are often employed simultaneously in large scale hydraulic fracturing operations. These pumps may be linked to one another through a common manifold, which mechanically collects and distributes the combined output of the individual pumps. For example, hydraulic fracturing operations often proceed in this manner with perhaps as many as twenty plunger pumps or more coupled together through a common manifold. A centralized computer system may be employed to direct the entire system for the duration of the operation.
However, the abrasive nature of fracturing fluids caused by the presence of proppant tends to wear out the internal components of the plunger pumps and associated piping components that are used to pump it. The repair, replacement and/or maintenance expenses for the internal components are extremely high, and the overall life expectancy is low for components used to convey fracturing fluids to the well bore.
To combat this state of affairs, pumping systems have been developed wherein a “dirty” stream of fracturing fluid (containing the abrasive proppant) and a “clean” stream of fracturing fluid (without proppant) is mixed in a common manifold at or in close proximity to the wellhead and delivered down hole to the zone to be fractured adjacent the wellbore. Each stream is supplied to the common wellhead manifold via a separate bank of positive displacement pumps. In such “split-stream” pumping systems, the excessive wear caused by entrained proppant is completely eliminated in the bank of pumps handling the “clean” fluids. Therefore, the extra maintenance is limited to the “dirty” bank of pumps.
An example of a split stream oilfield pumping system is disclosed in U.S. patent application Ser. No. 11/759,776 published under No. 2007/0277982 A1 on Dec. 6, 2007 to Shampine et al. Shampine however makes no provision for the use of recycled treatment fluid. Both of Shampine's fluid streams make use of new treatment fluid not previously recovered from the wellbore. Shampine moreover makes no provision for the use of fluids with a high Reid Vapor Pressure (RVP) that may or may not have been previously recovered from a wellbore.
Recycled treatment fluids demonstrate particular advantages for use in connection with fracturing operations. The recycling of fluid reduces the amount of new fluid required and the amount of fluid to be disposed of when the same fluid is used for a plurality of fracture treatments. Recycled treatment fluids present issues of their own however, which can limit their economic advantages. One of these issues is the Reid Vapor Pressure.
Under the ASTM Method D 323, Reid Vapor Pressure (“RVP”) is the absolute vapor pressure exerted by a liquid at 100° F. (37.8° C.). The higher this value, the more volatile the sample and the more readily it will evaporate. Unlike distillation data, RVP provides a single value that reflects the combined effect of the individual vapor pressure of the different petroleum fractions in a fluid sample in accordance with their mole ratios. It is thus possible for two wholly different products to exhibit the same vapor pressure at the same temperature—provided the cumulative pressures exerted by the fractions are the same. RVP plays a role in the prediction of hydrocarbon performance.
Recycled fracturing fluids typically have high RVP due to entrained hydrocarbons ingested when the fluids are pumped into and then recovered from oil and gas bearing formations. However, it is also contemplated that new (i.e. non-recycled) fluids may also have high RVP values. For this reason, the term “recycled” when used in reference to fluids will hereinafter refer generally to fracturing fluids having a high RVP value, regardless of whether the fluids are recycled or new. Some jurisdictions have regulations stipulating that such high RVP fluids for safety and environmental reasons are to be handled in a closed system to reduce or eliminate vaporization of the volatile hydrocarbon fluids, or the escape of the vapors to atmosphere. In the alternative, these high RVP fluids can undergo a re-conditioning process to remove the volatile hydrocarbon fractions as well as other substances. The need either to employ large scale containment systems or recondition the fracturing fluid prior to reuse, reduces the economic incentive to use them at all. Particularly, if a recycled fluid with a higher than acceptable RVP is to be blended with proppants, the size and expense of the containment system needed to enclose the proppant, the proppant auger and the blender is a significant disadvantage.
Accordingly, there is need for an oil field pumping system that can economically employ un-reconditioned recycled treatment fluids having high RVP without also requiring the use of large scale containment systems which would otherwise be necessary if the recycled fluid had to be blended with proppants.